1. Field of the Invention
The present invention relates generally to pattern recognition systems, and more particularly to pattern recognition systems with improved accuracy.
2. Description of the Prior Art
Pattern recognition systems of the prior art generally perform pattern recognition as shown in FIG. 1. An object 1 to be inspected is illuminated by light from a light source 3 powered by a stabilized power source 2. An electrooptical sensor 4, such as a television camera, senses an image of the object 1 and produces an electrical signal corresponding to the object 1 which is sent to a pattern recognition system 5.
Even though power for light source 3 is supplied by a stabilized power source 2, the light output of the light source 3 is not truly steady or constant over an extended period due to aging of the light source 3 and other causes. In addition, the ambient light level in the vicinity of the object 1 may change with time. Further, the video signal from camera 4 may exhibit changes due to drift, ambient temperature, component aging, etc. Such changes with time inevitably affect the judgment of the pattern recognition system 5. For example, even when the object 1 belongs to the same class as a standard object (in other words, when the difference between the two is smaller than a later explained predetermined value A), the difference G between the standard object and the object 1 being inspected may change with time in a manner shown by a curve a in the graph of FIG. 2. Further, when the object 1 being inspected does not belong to the same class as the standard object (in other words, when the difference between the two is larger than the predetermined value A), the difference G between the standard object and the object 1 being inspected also changes in a manner shown in a curve b in the graph of FIG. 2 as time passes. However, the predetermined value A which is used as a standard to judge whether or not the object 1 belongs to the same class as the standard object remains unchanged throughout the inspection process. Accordingly, in systems under the prior art, during period T1 of FIG. 2, while the object 1 actually belongs to the same class as the standard object, it is judged by the system 5 as not belonging to the same class. Further, during a time period of T2, even though the object 1 does not belong to the same class as the standard object, it is judged by the system 5 as belonging to the same class as the standard object and such misjudgment represents a fault of the system.